This invention relates generally to detecting defective cells in sensors including image sensors used in digital cameras, scanners and other electronic devices.
Referring to FIG. 1, a common digital image processing system includes a camera 100, a computer 102 coupled to the camera 100 by a communication link 104, and a display unit 106. The communication link 104 may be a serial bus such as the universal serial bus (USB). The display unit 106 may be any convenient display device such as a cathode ray tube (CRT) or liquid crystal display (LCD).
The camera 100 may use the imager 108 to generate an electrical representation of an optical image 110. To accomplish this, the imager 108 may include a sensor having an array of photon sensing elements. During an integration time or interval, each sensor element accumulates light energy from that portion of optical image 110 that is focused on it by camera 100 optics (not shown in FIG. 1). At the expiration of the integration interval, sensor elements indicate the intensity of the received light energy by, for example, an analog voltage signal. Camera 100 typically processes the indications from sensor elements to form a frame of digital data which may then be stored in memory internal to the camera 100 (not shown in FIG. 1), and/or transferred to the computer 102.
Image sensors are subject to defective pixels. However, because of the large number of pixels in image sensors, the fact that a few pixels are defective may not mean that the image sensor must be discarded. Instead, different manufacturers have quality standards which dictate that when the number of defective pixels exceeds a given number, then and only then, must the image sensor be discarded. These defective pixel numbers generally correspond to a number which adversely affects the quality of the image captured by the image sensor.
Generally, the pixel output signal is in the form of an intensity indication. A tester may determine whether the indicated pixel levels correspond to the expected intensities of the pixel. If not the pixel may be judged to be defective.
In addition to pixels that are deemed defective by virtue of their indicated intensity values, another type of defect which may affect the quality of an image sensor is a spatial defect. Basically, spatial defects are defects that arise due to the close proximity of two defective pixels. If two defective pixels are sufficiently close to one another, their combined effect may be additive. Thus, in addition to counting actually defective pixels, many image sensors are analyzed based on spatial defects. When the number of spatial defects and the number of pixel defects exceed a desired maximum, the image sensor may be considered unsuitable and may be discarded.
Image sensor test systems may capture a frame and then algorithmically determine which pixels are defective. The test system may be plugged into the image sensor. Generally the test system needs sufficient memory to hold and analyze the captured frame. Thus, the testers utilized for testing image sensors tend to be expensive. In addition, these testers consume algorithmic processing time. This algorithmic processing time is a function not only of the actual processing time but also the time needed to transfer the image data from the image sensor to the tester for external analysis. In addition, the transfer process itself may introduce noise which may further reduce the quality of the test results.
Thus, there is a continuing need for improved ways of testing sensors for defective sensing elements.